Device for measuring the thickness of sheet material



April 1954 J c. FROMMER DEVICE FOR MEASURING THE THICKNESS OF SHEET MATERIAL 22, 1950 5 Sheets-Sheet 1 Filed Dec.

JNVENTOR.

JOSEPH C. FROMMER Apnl 20, 1954 J. c. FROMMER 76, DEVICE FOR MEASURING THE THICKNESS OF SHEET MATER IAL Filed Dec. 22, 1950 5 SheeNs-Sheet 2 5 GING HEAD Ill FIG. 2

FIG. 4

R (BEING USED As REFERENCE PLATE) us n9 n7 PLUS MINUS STANDARD TOLERANCE ffoLEnANoE -||4 us L H3 1 L T- t 2 T 3 I T+ t T y REFERENCE PLATE H22 FIG. 3

V-GAGING HEAD I08 INVENTOR.

ROLLER (BEING USED AS JOSEPH R MMER REFERENCE PLATE FOR I By zs d W GAGING HEAD) v April 20, 1954 J. c. FROMMER 2,676,298

DEVICE FOR MEASURING THE THICKNESS OF SHEET MATERIAL Filed Dec. 22, 1950 3 Sheets-Sheet 3 7 I24 m n2 rrx I09 M no FIG. 6

FIG}? INVENTOR.

JOSEPH c. FROMMER ATTORNEY Patented Apr. 20, 1954 DEVICE FOR- MEASURING THE THICKNESS OF SHEET MATERIAL Joseph C. Frommer, Cin

to Electric Eye Equip cinnati, Ohio, assignor ment Company, Danville,

111., a corporation of Delaware Application December 22, 1950, Serial N 0. 202,383

Claims.

This invention relates to an apparatus for the detection of the thickness of materials in which the material to be tested is brought between a gaging head and a reference plate so that its thickness will influence the distance between them. The gaging head contains electrical means sensitiv to its distance from the reference plate. The output of these means is therefore indicative of the thickness of the material. This output can be amplified and used to indicate the thickness or to actuate control mechanism in accordance with the thickness.

When high accuracy of such a device is required, then it is customary to provide a second series with the second gaging head. Then a small percentage of change in the thickness to be measured will cause a substantial percentage of change in the resulting output, and slight changes in amplification will have a negligible efiect on indication or action ofthe device. The device can then be calibrated so that a certain deflection on an instrument corresponds to a certain deviation of the tested thickness from a standard thickness, the direction of the deviation being indicative of whether the thickness is below or above the standard. Similarly, one relay cr by another amount.

If, however, it is desired to detect the variation of thickness from one certain standard value in one case, and from some other standard value in some other case, then the calibration, giving deviation from standard as a function of meter deiiection, valid for the first case will ordinarily not be true for the second case.

Let us assume, for instance, that variations of the electrical capacitance between the reference plate and a surface on the gaging head is being used to detect their distance from each other.

The formula for this capacitance is F C K where C is the capacitance in micro-microfarads, K is the constant, T is the distance between said surface and the reference plate and F is the area of said surface on the gaging head.

The variation of this capacitance for an infinitesimal variation dT will then be:

Thus, if we increase the standard distance to twice its original value, the indication of the instrument would decrease to one-fourth of its indication for the same deviation at the former standard thickness.

A similar simple formula might be written for the case that the variation of the self inductance or of the mutual inductance of a coil, with the variation of its distance from a ferromagnetic reference plate is being used. But in this latter case, eddy currents, hysteresis losses and stray fiux make the real relationship substantially difshaped iron core of a certain transformer and a cast steel plate from .050" to .100" caused the mutual inductance between the two windings of the transformer to drop from about 39 millihenries to about 33 millihenries, and inductance to drop from 228 microhenries per mil to 84 microhenries per mil.

It is seen that whereas the usual method of bucking the output of the gaging head against that of a standard head is highly successful as long as the standard head is kept in one and the same tion each changed.

It is an object of the present invention to provide a device of the typ described in which calibration is independent of the standard distance between gaging head and reference plate, within a considerable range.

It is a further object of the invention to provide an improved device to detect the thickness of nonmagnetic materials.

Further object and advantages of the invention may be apparent in connection with its detailed description to be given herebelow.

To obtain an output which is indicative of the difference between the distance between the and its reference plate (to be called standard gap, as well as the distance between the gaging head and its reference plate is to be called test-gap) I provide a signal derived from the difiference of two gaps, the average value 01" these two gaps being held equal to or closely related to the value of the standard gap, the difference between them however being kept independent from the value of the standard gap. I vary the sensitivity of the heads or of the amplifiers connected to them automatically till the position, it requires an entire recalibratime the standard distance has to be proper, but the former splitting of th reference "both sides of the standard gap and allows to between the along the .line

amplifiers .e. g, type GAQE.

output from this constant gap-difference reaches a certain value. Then, if the standard gap is being increased in order to test thicker material, the output derived from difference between the test gap and the standard gap will be increased in the same proportion in which the sensitivity of the heads would decrease dueto the increased standard gap, and the calibration of the device will stay unchanged.

The constant gap difference necessary for this purpose may be obtained in various wail We may provide in addition to the standardlhea'dand the test-head, two additional heads, and link them mechanically with the standard head .in

such a manner that their'gaps vary by thesame amount by which the standard gap is being varied, keeping the gap of one-of themby-a-certain small amount above the standard .gap and and keeping the gap of the other below the standard gap by the same amount.

One of the two additional heads may be omitted by using thasignal of one such additional head bucked by the signal of the standard head way allows symmetrical gap-diiference toward match the two additional heads'with independen ly from the matching 'neces tween the standard head and the test-her 1.

Another way 'of obtaining the reference gapdifference is to varythe standard gap orthe test gap periodically by a given amount. Then one can keep the sensitivity of the device automatically at such a levelthat the alternations of the "1 other v beoutput occurring at the rate of said periodical variations stay constant independently of the average position of the periodically varied gap or the actual value of the other gap.

Other ways of obtaining the reference gapdifference may be devised within the scope of my invention. Whichever way this difference is provided, it can serve as a ready base of reference for the difference between the test-gap and the standard gap and will make the functioning of the device independent of all factors affecting the output from the constant gap difference in the same way-as theioutput from the difference test-gap and the standard gap.

My invention can be further understood by reference to the drawingslin which Figure 1 is a diagrammatic drawing of the electronic circuit used for controlling or indicating the thickness of the material being measured. Figure 2 illustrates-e. modification of the input portion of the circuit. vFigure 3 illustrates the standard gauging head and the two tolerance.gaugingheads. Figure i shows the exploring head in position for measuring the thickness of material on ardrum. Figure 5 is a cross-sectional view of Figure 4 Figure representsarnodification of Figur l illustrating an exploring head on an eccentric carriage. Figure F! is a crosssectional View of Figure 6 along the line L-l.

In Fig. 1 tube to is a conventional. generator of alternating voltage, which generates ranalterhating voltage of say 409 cycles per second. It will benoted that itsgridsare connected across resistors l5, it to plate M of detector tube Hi, the function of which will be cl scribed later. Theoscillation generated in tube iii is impressed on grids =2-.!,, 3! of amplifier tubes 2t and 36. These .tubeslare preferably beam power vacuum.

These tubes are connected in a conventional push pull circuit and feed an alternating voltage of say-400 cycles per 'around an iron core.

It is therefore being kept second to the transformer 26. winding of transformer 2.6 feeds the primaries HI, I20, H6 and H5 of the four transformers l, 2, 4'. which are all connected in series with each other. These transformers have each a primary and .a secondary winding, both wound This ironcore is situated opposite an iron plate. Transformer 3 is the sensitive element of the standard is the one of the test gaging head and l and 2 are ltheiones of the two auxiliary gaging heads. fi ls the four primaries are all connected in series with eachv other, anddue to the fact that all four .primarieshave thesame number of turns, the loo cycles .persecond magnetornotive force generated in all four of these transformers will be exactly equal. The 'magnetic flux generated by this magnetomotive force depends however not only on the magnetomotive force in the primaries, but also on the magnetic reluctance of the path which links the "primary with the secondary. This reluctance "is substantially dependent on the air gap between the iron cores and the co ordinated steel plates. Accordingly, the .400 cycles persecond flux and the electromotive force generated in the secondaries will depend on the distance between iron core and plate and it is this dependence which is being used to indicate the distance between iron core and metal plate.

The secondaries H 2, Hi of transformers 3 and d are connectedin series with each other in such a phase relationshipthat the 'electromotive forces in these'two secondaries tend to cancel out each other. If, therefore, the distance between iron core and steelplate of systems 3 and are equal andall other factors "are equally liept symmetrical, these'two E. M. Fs. will cancel each other out completely and no signal whatsoever will appear 'at'the gridlil of amplifier tube 56.

In a similar manner, the outputs of coils H! and H9 of transformers l and 2 would cancel out .each other, "if the "respective distances were exactly equal. It was p'ointed out, however, that systems I and? are kept constantly at different distances from each other and so the signal on grid 5| of tube-4H will never become zero. The signal from '1 and 2 is 'aplified by tube 45 and preferably is further amplified by means not shown and causes a .correspondingdy (amplified signal 'in the transformer 45 connected to its plate.

Similarly the signal obtained on grid 5i is amplified and appearsin'transformer so. It will he noticed that the center tap of secondary of 51E iis'connec'ted to one end of the secondary or at alternating voltage depending in-its phase on the phase of the voltage impressed togr id '41. Accordingly the two terminals of this secondary of 55 will assume voltages which are the sum of the secondary voltage in 46 and the respectivehalf of the secondary voltage .in Till. As these two halves are in opposite polarities, I obtain on one terminal substantially the diiference and on the other terminal the sum of the respective secondary electromoti-veforces. Therefore, as long as the standard distance and exploring distance (3 and 4) are exactly equal, bothends of the secondary of 56 will be at the same alternating potential (because the alternating voltagein the two halves is now zero). But if the distance of the explorzing system changes, than we obtain on one end-a The secondary higher candzon'the other end za lower (.400 cycles gaging head, t

ard distance. The alternating voltages obtained on these two terminals are impressed to plates 64 and 64' As long as these The voltage obtained on diode plates 64, 64 is filtered by resistors 65, 65' and capacitors 66 and 65' and applied to grids 8 I, 8 I of twin amplifier 80. This twin triode amplifier (e. g., a type 12AU'7 tube) is connected in the well known cathode followlay 9'! in its plate circuit and having its grid 9| connected across filter network 61, 68 to plate 64. This grid is connected further across resistor 95 to the tap on a potentiometer 96. Accordingly voltage dependalso for deviation in the opposite direction, then one can add a tube I00, relay I01 to act in a similar manner for deviations which will make plate 54 more negative.

The system as described cate the direction of deviation,

been set from its plate. Indeed it is readily seen that if the airgap on the standard is set to .100", .001" deviation will cause a substantially lower E. M. F. than it would when the standard is being set to say .025" from its plate.

M is connected to grid resistors I 5, I of oscillator tube I0. ii is fed from the secondary of transformer 45. Anode I4 will assume a voltage which Now if this alternating voltage causes in the diode system H I4 a voltage substantially higher than 150' volts then there will appear on the grid resistors I5, I5 a high negative bias, which tends to diminish the oscillations of the tube I0. If the oscillations of this tube diminish, then the output of tubes 20, 30 will diminish accordingly and the magnetomotive force in all four transformer primaries of transformers I, 2, 3, I! will diminish. If the magnetomotive force in I, 2, diminishes. the signal in 6 tube 40 diminishes, which in turn will diminish the A. C. impressed on system H, I4, bias generated by this the voltage rectified in system H, i4 is just slightly above volts. And if the standard is being set to a different distance, the same mechanism as just described will again work until the output of II, 14' is again slightly above 150 volts. Thus by this means, the difierential that exists between I and 2 will cause a constant output irrespective of whether the air gap on each of them is small or large. Now if the auxiliary coils I, 2 are kept substantially at the same distance from their respective plates as is the between them (e. an, if the standard is .025, we have I and 2 at .020 and .030" respectively. If the standard coil is at .100", we keep I and 2 at .095" and .105" respectively). Thus we always have an output of 150 volts for say .010" between auxiliaries. Now if the main amplifier system (tube 50 and transformer 56) is substantially identical with the auxiliary amplifier comprising the tube 40 and the transformer 40, then one also will get in the main amplifier 150 volts for a diiferential of .010" between standard coil and exploring coil, irrespective whether the standard is being set to .025 or .100" or whatever other value.

drawing gives only the most substantial portions of the circuit. In the actual circuit, I apply the automatic control from I4 not to the oscillator but to supercontrol amplifier tubes connected between this oscillator and the power tubes 20, 30. I also prefer to have one or more stages of amplification between tubes 40 and transformer 46 and tube 50 and transformer 56 respectively. I have reduced the drawing however to the simplest form for better understanding.

It also should be of the main coils (3 or 4 to serve instead of uses core I is not required and a cheaper and less cumbersome device is thereby obtained.

In Figure 4, gauging head I08, including coils III and H2 (also shown in Figure 1) and core 4 is in position to measure the thickness of paper I09 on drum IIO.

Referring to Figure 3, a standard gauge H4 is provided, having a core 3 and coils I20 and IZI, also shown in Figure 1. Likewise, the tolerance or auxiliary gauging heads H3 and H5 have cores 2 and I, respectively, and coils I10 head Il3, and coils II6 and H9 for Reference plate I22 is provided in order to have a reference point in setting the distances for the respective gauges.

It should be understood that a standard differential at the distance of the standard can be obtained in other ways, too. One would be to th cylinder I 24 on being mounted can adjusting the sensitivityof the system always to give readings for equal deviation between standard (see Figure 0) and; the unknown (Figure 6).

My invention can be further understood by describing a typical operation of my device. First, gauge H4 is adjusted so that core 3 is at a distance T from the reference plate 122 which distance is equal to the standard thickness of the material to be measured, plus the distance between the lower edge of core 4 and the upper surface of the material H19 being measured as indicated in Figure 4.. Core 2 is adjusted to a distance, T t, which distance serves as a reference for controlling the minus tolerance. Likewise, the plus tolerance is set by adjusting core 1 to a dis tance, T plus it, which serves as a reference for determining the plus tolerance. The gauging head I08 is placed on the material being gauged Hi9, say paper and if the thickness of the material being gauged is equal to the standard thickness, the signal obtained from series connection of the respective coils on core 3 and core 4 equals zero. Accordingly, tube 50 obtains zero input signal and the alternating voltage of this system will be zero.

However, if the material N39 is less than the standard thickness, then the input signal to tube 58 is going to be an alternating voltage in-phase with the input signal to tube til which will cause relay ill! to be energized which, in turn, can control a circuit for adjusting the thickness. Furthermore, the current across meter til will have a polarity and magnitude indicative of the amount of deviation from the standard. Lil. .e wise when the paper 109 is thicker than the standard thickness, tube 50 obtains a signal opposite in polarity to the signal of the tube Mi which will cause relay 91 to be energized. This relay may control the necessary controlling mechanism. Furthermore, the current across meter 33 will assume a direction opposite to the one when the paper was thinner than the standard.

The above description of the drawings and of the typical operation of my device are only illustrative of my invention and other modifications may be made within the scope of the following claims.

I claim:

1. A device for determining the thickness of materials by means of a standard gaging head and an exploring gaging head comprising in combination separate means to generate electrical signals depending on the distance between two portions of said gaging heads, said signals being connected in opposition so as to yield an output substantially dependent on the difierence between the distances sensed by said two gaging heads, means to adjust said standard gaging head to a predetermined reference distance from a reference plate, means to influence said exploring head by the thickness to be measured. a pair of auxiliary gaging heads mechanically linked with one of said gaging heads such as to vary their respective distances from said reference plate simultaneously with variations in said distance of said gaging head, means maintaining the difference between their distances at a predetermined constant value, the outputs of said two, auxiliary gaging heads being also connected in opposition, means to simultaneously influence the sensitivity of all four of said gaging heads being also connected in opposition, means to simultaneously influence the sensitivity of all four of said gaging heads connected to said output of said pair of auxiliary gaging heads including means for maintaining said output of said pair of auxiliary gaging heads at a substantially constant level.

2. A device according to claim 1 in which the exploring gaging head is used as one of the two auxiliary gaging heads thereby serving a dual function.

3. A device according to claim 1 in which the standard gaging head is used as one of the auxiliary gaging heads thereby serving a dual function.

A. device for determining the thickness of materials comprising a standard gaging head, an exploring head and reference plates, means maintaining said standard head a predetermined distance from one of said reference plates, means causing the distance between said exploring head and another of said reference plates to vary with the thickness of the material measured, the outputs of said heads connected in opposition so as to obtain a signal, the phase and magnitude of which is dependent on the direction and magnitude of deviation between the standard and unknown, means causing a known deviation around the distance of one of said heads from its respective reference plate and mechanically linked to said one of said heads, and automatic means for causing the output from said known deviation render the aforementioned signal independent of the said predetermined distance of said standard head from its reference plate.

5 a devi for determining the thickness ials comprising a standard gaging an e pioring head and reference plates, maintaining said standard head a predetermined distance from one of said reference plates, means causing the distance between said exploring head and another of said reference plates to vary with the thickness or the material measured, the out puts of said heads connected in opposition so as to obtain a signal, the phase and mag? tude of which is dependent on the direction and magniof deviation between the standard and unknown, means causing a constant periodical deviation of the distance of one of said iiieads from its respective reference plate automatic means causing the periodical deviation of the obtained signal caused by said constant periodical deviations of the distance of one of heads from respective reference plate to stay substantially constant and causing thereby the sensitivity of the device toward variations of the thickness of the material measured to remain so. tantially constant.

References Cited in the file of this patent UNITED STATES PATENTS 

